The present invention relates to a gas-feeding nozzle for use with an injection molding machine for producing an injection-molded article which is free of sink marks and distortion and is excellent in appearance. More specifically, it relates to a gas-feeding nozzle for introducing a pressurized gas into a molten resin injected into the cavity of a mold when a molded article having a hollow structure is produced by means of an injection molding machine.
For example, JP-A-64-14012 (corresponding to U.S. Pat. No. 4,740,150) discloses an injection molding machine used for producing a molded article free of sink marks and distortion and excellent in appearance. In the technique disclosed in JP-A-64-14012, a molten thermoplastic resin is injected into the cavity of a mold, then a pressurized gas is introduced into the molten thermoplastic resin in the cavity to form a hollow structure within the thermoplastic resin, and the gas within the hollow structure is released into atmosphere before the mold is opened.
The above JP-A-64-14012 also discloses a valve mechanism for introducing a pressurized gas. This valve mechanism is provided with a bore, and the pressurized gas is introduced through the bore into the molten thermoplastic resin injected into the cavity of the mold. A non-return valve is provided in the outlet end of the bore. The non-return valve prevents the molten thermoplastic resin injected into the cavity from flowing into the bore. Further, after the pressurized gas is introduced into the molten thermoplastic resin injected into the cavity, the non-return valve prevents the pressurized gas from flowing back from the hollow structure formed within the molten thermoplastic resin to the bore while the thermoplastic resin is cooled to solidness in the cavity.
However, when molded articles are produced according to the technique disclosed in the above JP-A-64-14012, the following problem is liable to occur. When the gas within the hollow structure is released into atmosphere, resin fragments are blown off to adhere the periphery of the non-return valve of the valve mechanism, or the resin remaining in the bore adheres to the non-return valve. As a result, the non-return valve can no longer function properly when a molten thermoplastic resin is injected into the cavity, and part of the molten thermoplastic resin flows into the bore through the non-return valve and further flows into a tubing for feeding a pressurized gas (sometimes referred to as "gas tubing" hereinafter).
When the molten thermoplastic resin has flown into the bore or the gas tubing, the bore or the gas tubing may be cleaned by a method in which the molten thermoplastic resin is pressed back to the cavity by means of a pressurized gas. However, it is very difficult to remove the molten thermoplastic resin completely from the bore or the gas tubing, and cooled and solidified resin builds up within the bore or the gas tubing. As a consequence, the resin that has built up as above is liable to clog the bore and the gas tubing, and finally prevents the introduction of a pressurized gas into a molten thermoplastic resin injected into the cavity.
For example, JP-A-l-157823 (corresponding to U.S. Pat. Nos. 4,855,094 and 4,943,407) discloses the use of an orifice having a diameter of 0.13 to 10 mm as a nozzle for feeding a pressurized gas in an injection molding machine for producing molded articles having a hollow structure. However, when molded articles are produced according to the technique disclosed in this JP-A-l-157823, the orifice is sometimes clogged depending upon the kind and viscosity of a resin used and injection molding conditions.